The high-intensity discharge (HID) lamps have been widely used in many applications because of their high efficiency, good color rendering and long life span. However, the HID lamp is a complex load, the lamp parameters (voltage, current, power etc.) frequently change during its operation time. FIG. 1 shows a typical control strategy for the discharge lamps in the prior art. After ignition, the discharge lamp is usually operated at a constant current mode during a run-up stage and the discharge lamp power increases gradually with the increasing of discharge lamp voltage (Vlamp). When the discharge lamp voltage is higher than a first predetermined value V1 the discharge lamp power enters a stable working status—a constant power stage so as to attain better discharge lamp performance. The discharge lamp is judged to be at the end of its life when its voltage is higher than a second predetermined value V2, and it is switched off when its voltage is higher than a specified value. Therefore, a special power supply named ballast is then needed for these discharge lamps. FIG. 2 shows a block diagram of the ballast circuit for a HID lamp with power factor correction (PFC) circuit as the first stage in the prior art. And the second stage—the DC/AC inverter, inverts the output of the PFC circuit to a voltage required by the HID lamp. The controller adopts proper control method so as to realize the typical control strategy shown in FIG. 1. Besides, in FIG. 2, the ballast circuit further includes an AC power source, an electromagnetic interference (EMI) filter and a rectifier, wherein the EMI filter receives the AC power source and the rectifier connected between the EMI filter and the PFC circuit.
FIG. 3 shows a schematic circuit diagram of a ballast circuit in the prior art, in which only the DC/AC inverter of the second stage, the controller and the discharge lamp are shown. And the DC/AC inverter is a half-bridge circuit acting as a double down-converter. The double down-converter includes a first MOSFET S1, a second MOSFET S2, a first and a second body diodes D1 and D2, an inductor L2 connected to the discharge lamp in series, a capacitor C2 connected to the discharge lamp in parallel and two electrolytic bridge capacitors CH1 and CH2 connected in series. The double down-converter is operated in the critical continuous mode with the controller, e.g., L6562. In each half commutation period (commutation frequency is in the order of 100 Hz), one MOSFET (S1 or S2) operates in higher frequency, e.g., 100 kHz, in combination with the diode (D2 or D1) of the other MOSFET as a Buck converter. The resistive divider of R1 and R2 is used to sense the value of discharge lamp voltage. C3 acts as a noise filter. Equation (1) shows the relationship between the sensed discharge lamp voltage VC and the real discharge lamp voltage Vlamp.VC=(VDC/2±Vlamp)*R2/(R1+R2)  (1),
VDC is the output voltage of the PFC circuit, and Vlamp is the discharge lamp voltage.
Based on the sensed discharge lamp voltage VC, the micro controller unit (MCU) then outputs a control signal to a first controller—the DCMB (discontinuous conduction mode boundary) controller to adjust the duty ratio of the driving signal of the MOSFET S1 and S2 so as to achieve the power regulating and the detection of end of the life according to the typical lamp control strategy.
One major drawback of the prior art is that the resistive voltage divider suffers from high voltage stress. And the usage of the resistive voltage divider increases the cost and reduces the power density of the ballast converter. As can be seen, before the lamp ignition, the maximum voltage across resistors R1 and R2 equals to the voltage Vlamp plus half of the voltage VDC of the output of the PFC circuit. Assuming that VDC=450V, then Vlamp=225V (generally before ignition Vlamp=VDC/2), therefore, the resistive voltage divider have to endure a voltage rating of at least 450V.
From the above analysis, a new scheme is then needed to overcome the drawbacks of the prior art for sensing the discharge lamp voltage.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a voltage sensing apparatus for the power regulation and the over-voltage protection of a discharge lamp system and a controlling method thereof.